Question 150035
The simplest explanation is because someone said so.
From exponentiation, you know the division rule that,
{{{x^M/x^N=X^(M-N)}}}
If M=N, then
{{{x^M/x^M=x^(M-M)}}}
{{{1=x^(0)}}}
If it was not the case, then that example wouldn't hold up. 
You would disprove the division rule. 
Your mathematical system would fall apart. 
That definition is that way so that your mathematical system remains sound and consistent. 
That's a simple answer, but it's actually a very complicated question.